The need to improve joint stiffness and to reduce cost and weight are critical to the efficient and effective use of aluminum in automotive and aircraft applications. Adhesive bonding as a method of joining is useful for accomplishing these objectives. However, achieving a durable adhesive bond often requires using an aluminum surface treatment. Traditional aluminum surface treatments designed for this purpose employ chromium-containing chemicals. Environmental concerns about chromium may soon render these surface treatments obsolete.
The present invention relates to a non-chromium "pretreated" aluminum sheet product. As used herein, the term "pretreated" refers to a surface treatment applied to an aluminum sheet before it is coiled for shipment to a user. A process for pretreating aluminum sheet utilizing a chromate solution is disclosed in Selwood et al U.S. Pat. No. 5,026,612.
The term "sheet" refers to aluminum alloy material having a thickness of about 0.006 to 0.249 inch. The term "plate" refers to aluminum alloy material having a thickness of 0.25 inch or more.
The primary criterion for selecting an aluminum pretreatment is adhesive bond durability. It is also imperative that the surface treatment not be detrimental to other downstream processes. For example, the surface treatment must not inhibit stamping and forming of the sheet. The surface treatment must remain intact during those operations. Resistance spot welding will be used in combination with adhesive bonding to improve performance in peel strength and in fixture components until the adhesive is cured. For that reason, it is important that the pretreatment not significantly reduce the quality of the welds or the life expectancy of the welding electrodes. It is also critical that the pretreatment not be detrimental to the painting process or to the chemical baths required for zinc phosphating and electrocoating before paint is applied.
A principal objective of the present invention is to provide a process for coating aluminum alloy sheet to provide durable adhesion to an adjacent component through a polymeric adhesive.
A related objective of the invention is to provide a non-chromate organic coating for aluminum alloy sheet that will not interfere with the quality of welds or reduce the life expectancy of metal welding electrodes.
Additional objectives and advantages of our invention will be readily apparent to persons skilled in the art from the following detailed description of a preferred embodiment.